Certain types of test equipment, such as logic analyzers and timing analyzers, have a large number of input data channels. Each data channel has a probe associated therewith. Some probes are a complete apparatus in themselves, perhaps including attenuators and active electronics. Others are nothing more than a simple length of wire extending from a probe pod and having an individual push-on connector at a distal end for connection to a signal of interest; the probe pod contains any needed signal conditioning and threshold detection circuitry. In certain cases where the data to be probed occurs within a well defined environment, such as bus, what can extend from the probe pod is a suitable multi-conductor cable having a standard multi-pin connector mounted at its distal end.
In a case where probing is accomplished by attaching a standard multi-pin connector there is little or no chance for an error concerning the correspondence between particular data signals in the environment under test and the particular channels available on the test equipment; it is literally hardwired. However, the case where an operator has to manually place a large number of individual probes (which we may term the "do it yourself case") is sometimes the occasion for confusion and/or error. The sheer number of connections to be made, in conjunction with tight quarters and tiny printing (not to mention last minute changes in ones plans) afford ample occasion for confusion, which may or may not also be accompanied by error.
In any event, the operator generally has a pretty good idea of which channel should go where in the environment be measured. So, when holding a probe yet to be attached, or when holding a probe that has fallen off and needs to be re-attached, the first question likely to come to mind is "What channel is this?" Some oscilloscopes have approached this problem by placing a channel ID push button in a place easily associated with the probe. The activation of the ID button shifts or intensifies that portion of the display that is for the associated channel. The location of the ID button could be on the front panel of the scope itself, which works as long as the number of probe cables is small enough that the eye can follow a cable from one end to the other (which is to say that probe-to-channel correspondence is not a difficult issue, anyway). The location of the ID button could be on the probe body, but that is a real complication in a system with thirty-two (or more!) probes. (There are cost and reliability issues surrounding the extra conductors and their connections, as well as the ugliness of the added physical size needed to support a switch.) All in all, the real value of such an ID button is in an oscilloscope setting of say, eight channels or less, where each channel is independently positionable on the screen. That is, at this moment the D channel might be at the top of the screen, whereas ten minutes ago the A channel was on top. The operator eventually forgets which state of affairs is presently the case, and resorts to the channel ID buttons to learn the order in which the channels are positioned on the screen. Probe-to-signal correspondence is generally not the issue, since the operator can usually see what it is simply by looking, because the number of channels is small. If the number of channels were large, then an ID button would have to be on the probe to be effective, but the associated design issues are unpleasant to contemplate.
While state analyzers and timing analyzers do not generally have (fine) vertical position controls for individual channels, they do usually allow the user to specify which trace represents which channel (an ordering of the channels, which amounts to a coarse position control), so the positioning of channels on the screen remains a potential source of mischief. They also often have a large number of channels and associated probes. Probe-to-channel correspondence can thus be a very serious issue, indeed. Individual ID buttons on the probes are even less desirable in this setting, since the probes are often simply short pieces of wire, far smaller than even the most compact attenuator probe for an oscilloscope.
The usual solution for this problem is color coding of the probe wires and labelling on the probe pods. The problem with that solution is that wires coming out of the probe pod are usually replaceable (for various reasons) and in any given instance may not be correctly coded by color according to their location on the probe pod. Even if they are, it is easy to incorrectly parse the colored stripes or bands, and some individuals do not correctly perceive certain colors in the first place. Still other persons have never learned the color code. Labels on the probe pod are useful only when it is possible to follow the wire from one end to the other, a task that can be difficult in a "bird's nest" of wires. Thus, there is a need for certain types of test equipment to be able to assist an operator in answering the question "Which channel is associated with this probe?"